[en] It is important to clarify the mechanism and to prevent the stress corrosion cracking (SCC) in order to control the water quality of primary cooling water in LWRs. It is also necessary to measure the concentration of some corrosive chemical species of the structural materials such as oxygen and hydrogen peroxide in the cooling water. Generally, the concentration of oxygen and hydrogen peroxide in the cooling water has been measured with sampled water outside the reactor, and the corrosive environment has been estimated. However, it is difficult to evaluate the accurate concentration of the corrosive chemical species by this method, because the species are varied due to the high temperature and pressure under neutron irradiation condition. Therefore, the in-situ water analyzer has been developed for the concentration measurement of corrosive chemical species. The analyzer is composed of the spectrometer as well as chemical sensor, and the concentration can be evaluated by the intensity of the absorption and luminescence of the chemical species in the reactor water. In this study, the preliminary performance test of the in-situ water analyzer was carried out under the normal temperature and pressure, and optical transmission through the optical fiber was measured with the guide tube. (author)